Article pubs.acs.org/IECR
Effects of Metal Ion Type on Ionomer-Assisted Reactive Toughening of Poly(lactic acid) Hongzhi Liu, Xiaojie Guo, Wenjia Song, and Jinwen Zhang* Composite Materials and Engineering Center, Washington State University, Pullman, Washington 99164, United States ABSTRACT: In this study, toughening of poly(lactic acid) (PLA) through reactive blending with epoxy-containing elastomer and ionomer was investigated. Four commercial ionomers based on the poly(ethylene-co-methacrylic acid) (EMAA) precursor were evaluated in this study, which contained zinc, magnesium, sodium, an lithium ions, respectively. Effects of metal ion type, elastomer/ionomer weight ratio and blending temperature on impact toughness of PLA ternary blends were studied. The toughening effects of metal ions of the ionomers is in the order of Zn > Mg > Li ≈ Na. High blending temperature and high elastomer/ionomer ratio both promoted the effective toughening of the PLA ternary blends. The reactive compatibilization and cross-linking of the epoxy-containing elastomer were analyzed using FT-IR, SEM, dynamic mechanical analysis, and torque rheology. In addition, the effect of metal ion type on thermal degradation of PLA was also examined.
1. INTRODUCTION Poly(lactic acid) (PLA) is a very promising biodegradable polyester derived from renewable resources. In recent years, PLA toughening has attracted considerable attention. So far, the main routes for PLA toughening involve addition of plasticizers,1,2 copolymerization,3,4 addition of rigid fillers,5 and melt-blending with various flexible polymers.6−15 Of the aforementioned strategies, reactive blending with rubbers or elastomers has been shown to be especially effective in achieving supertoughness.10,11,15 In our previous study, a novel PLA ternary blend system consisting of ethylene/nbutyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer elastomer and zinc ionomer (containing Zn2+ ions) of ethylene/methacrylic acid copolymer (EMAA-Zn) was reported.15 The unique feature of this ternary blend system lies in the fact that both interfacial compatibilization of PLA/EBAGMA and cross-linking reaction of EBAGMA triggered by EMAA-Zn occur simultaneously during reactive blending. The balance between both reactions was found to be critical in achieving desired toughening effects.16 Further studies also showed that reactive blending temperature,15,17 EBA-GMA/ EMAA-Zn ratio,16,17 and the characteristics of EMAA-Zn (i.e., neutralization degree of MAA and MAA content of the ionomer precursor),18 all had significant effects on the impact toughness of the ternary blends. In our study, at the optimum blend composition and elevated blending temperature, a notched Izod impact strength (IS) up to 860 J/m was successfully achieved.15 The free carboxylic acid groups in the ionomer can initiate the curing of epoxy groups in the EBAGMA phase, while zinc ions can catalyze the interfacial compatibilization between the pendent epoxy groups of EBAGMA and end functional groups of PLA at elevated blending temperatures, thereby resulting in improved interfacial wetting.15 Other investigators have found that some metal salts and complexes can catalyze the curing of epoxy resins, and that the reactivity depends largely on the type of metal ions.19 It is thus reasonable to anticipate that the kinetics of the two reactions which both involve the epoxy functional groups will also depends on the metal ion type of the ionomers in our © 2013 American Chemical Society
ternary PLA blend system. On the other hand, it is known that PLA is susceptible to thermal degradation during molten processing (especially at elevated temperature and long residence time), which adversely affects the mechanical properties of the final products.20,21 Other studies showed that many residual metal (e.g., Zn, Mg, Al, Fe, Ti, Zr, Sn, and Ca) compounds markedly affected thermal degradation behaviors of PLA, and their catalytic activity on PLA degradation varied with the metal ion involved.22−24 Therefore, another motivation of our investigation arose from the assumption that the type of metal ions in the ionomers may play an important role in the thermal degradation of the PLA matrix, presumably influencing the phase morphologies and mechanical properties of the PLA ternary blends. In this work, in addition to the previously reported EMAAZn ionomer, we selected three other EMAA-derived ionomers respectively containing Li+, Na+, and Mg2+ ions (which we denoted as “EMAA-Li”, “EMAA-Na”, and “EMAA-Mg”, respectively) to prepare the ternary blends. The impact toughness and phase morphology of the ternary blends were compared. To account for the difference in toughening effect between these ionomers, the influences of metal ion type in the ionomers on interfacial compatibilization, cross-linking of elastomer, thermal degradation, and crystallinity of the PLA component in the ternary blends were investigated.
2. EXPERIMENTAL SECTION 2.1. Materials. All materials used in this work are commercially available. NatureWorks PLA 2002D with a melt index (MI) value of 5−7 g/10 min (210 °C, 2.16 kg) was used for the blends. Ethylene/n-butyl acrylate/glycidyl methacrylate terpolymer pellets (Elvaloy PTW; designated as “EBA-GMA”) with a MI value of 12 g/10 min (190 °C, 2.16 kg), was kindly Received: Revised: Accepted: Published: 4787
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compounding, a torque rheometer (Haake Rheomix 600p) was utilized to monitor the mixing torque under the same mixing speed and temperature as the extrusion blending. The polymer mixture was charged into the mixing chamber as soon as the rotors began to rotate. Upon the completion of feeding, the inlet of the chamber was immediately closed and the mixture was allowed to mix. The torque value was recorded as a function of mixing time. 2.3.5. Gel Permeation Chromatography (GPC). The filtrate of the above extraction was poured into excessive cold methanol to precipitate the PLA component. The precipitates were collected by centrifugation and then dried under a vacuum prior to GPC analysis. The absolute number-average molecular weight (Mn) and molecular weight distribution (i.e., polydispersity index, PDI = Mw/Mn) of the above PLA isolates from the ternary blends were determined using an Agilent 1100 system (Agilent 1100 series degasser, isocratic pump and autosampler) equipped with two Phenomenex 10 μm, 300 × 7.8 mm columns [105 Ǻ and MXA], Wyatt DAWNEOS multiangle light scattering (MALS) detector (He−Np 5 mW laser at λ = 632.8 nm), and an Agilent 1200 differential refractive index (DRI) detector. Tetrahydrofuran was used as the eluent at a flow rate of 1.0 mL/min. The detector temperature was 25 °C. The samples were dissolved in chloroform with the concentration of 20 mg/mL and injected by 25 μL. 2.3.6. Differential Scanning Calorimetry (DSC). DSC tests were conducted on the injection molded samples using a Mettler Toledo DSC 822e under nitrogen atmosphere. An approximate 5 mg sample taken from the injection molded specimens was heated to 200 °C at a heating rate of 10 °C/min. The crystallinity of PLA (Xc) in the injection molded specimens was determined from the first heating scan using the following equation:
provided by DuPont Co. and its E/BA/GMA monomer ratio is 66.75/28/5.25 by weight.25 All four commercial ionomers containing different metal ions (i.e., zinc, magnesium, lithium, and sodium) were derived from the ethylene/methacryalic acid copolymer (EMAA) precursors and had almost identical contents of MAA monomer (i.e., 14.7−15.2 wt %).26,27 The neutralization degree was about 40% for the lithium, sodium, and zinc-containing ionomers 26 and was 55% for the magnesium-containing ionomer.27 These four ionomers corresponded to Surlyn 9945, Surlyn 6910, Surlyn 7940, and Surlyn 8945 under the commercial grade, respectively, and were also kindly provided by DuPont Co. 2.2. Test Specimen Preparation. Melt blending was performed using a corotating twin screw extruder (Leistritz ZSE-18), with a screw diameter of 17.8 mm and a L/D ratio of 40. The screw speed was set at 50 rpm and the melt zone temperatures were varied to be 185, 210, and 240 °C, respectively. For all ternary blends, the total content of PLA was fixed at 80 wt %. After being oven-dried at 80 °C overnight, the extruded pellets were injection-molded into standard tensile (ASTM D638, Type I) and Izod impact (ASTM D256) test bars using a Sumitomo SE50D injection molding machine at a barrel temperature of 190 °C and a mold temperature of 35 °C. 2.3. Characterization. 2.3.1. Tensile and Impact Testing. Notched Izod impact tests were performed according to ASTM D256-05 using a BPI-0-1 Basic Pendulum impact tester (Dynisco, MA). The injection molded impact bar samples were notched using a XQZ-I specimen notch cutter (Chengde Jinjian Testing Instrument Co., Ltd., China). The V-shape notch had an angle of 45 ± 1°, a depth of 2.54 ± 0.05 mm, and a radius of curvature at the apex of 0.25 ± 0.05 mm. Tensile tests were conducted on an 8.9 kN, screw-driven universal testing machine (Instron 4466) equipped with a 10 kN electronic load cell and mechanical grips according to the ASTM 638 standard. The crosshead speed was 5.08 mm/min, and the initial strain was measured using a 2-in. Model 35420200-010-ST extensometer (Epsilon Technology Co., WY). An average value on five repeats was taken for each sample. All tensile and impact test specimens were conditioned for 7 days at 23 °C and 50% relative humidity (RH) prior to mechanical tests and other characterizations. 2.3.2. Fourier Transform-Infrared Spectroscopy (FT-IR). Spectra were recorded in the absorption mode using a Thermo Nicolet Nexus 670 spectrometer with a resolution of 4 cm−1 and 32 scans. For each blend, the injection molded specimen was sliced to thin films of ∼120 μm in thickness using a microtome, followed by 1,4-dioxane extraction under stirring at ambient temperature for 10 days to selectively dissolve the free PLA component. The insoluble residue was vacuum-dried, ground with dry KBr powder and then compressed into discs for FT-IR testing. After the baseline correction, deconvolution of the peaks at ∼1759 cm−1 and ∼1734 cm−1 was established using the Lorentzian function. 2.3.3. Dynamic Mechanical Analysis (DMA). Dynamic mechanical properties of the blends were measured using a DMA Q800 analyzer (TA Instruments). The test was conducted under the single-cantilever mode and an oscillating frequency of 1 Hz. The temperature was scanned from −100 to 150 °C at a heating rate of 3 °C/min. For each sample, duplicate tests were performed to ensure the reproducibility of the data. 2.3.4. Torque Rheology. To evaluate the cross-linking reaction between EBA-GMA and ionomer during molten
Xc % =
ΔHm − ΔHc × 100 wf × ΔHmo
(1)
where ΔHm and ΔHc are the enthalpies of melting and cold crystallization during heating, respectively; ΔHm° is the enthalpy assuming 100% crystalline PLA homopolymers (93.7 J/g),28 and wf is the weight fraction of PLA component in the blends. 2.3.7. Transmission Electron Microscopy (TEM). The phase morphology of the blends was examined by transmission electron microscopy (TEM, JEOL 1200EX) at an accelerated voltage of 100 kV. Ultrathin sections (60−80 nm in thickness) were cryogenically sliced at −80 to −100 °C using a RMC CRX microtome equipped with a diamond knife and placed on carbon/Formvar coated 200-mesh nickel grids. For the purpose of particle size analysis, at least 600 particles from four independent TEM images were analyzed by a semiautomated image analysis technique based on NIH image software. The cross-sectional area (Ai) of each individual particle (i) was measured and converted into an equivalent diameter of a sphere with the equation di = (4Ai/π)0.5. 2.3.8. Scanning Electron Microscopy (SEM). The freshly fractured surfaces in liquid nitrogen were sputter-coated with a thin layer of gold and then examined for interfacial wetting with a Quanta 200F field emission scanning electron microscope (FE-SEM, FEI Company) at an accelerated voltage of 15 kV. 4788
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3. RESULTS AND DISCUSSION 3.1. Impact Properties and Thermal Degradation of PLA. Figure 1 illustrates the effect of blending temperature on
Figure 2. Effect of ionomer content on impact strength of PLA/EBAGMA/EMAA-M (80/20 − x/x, w/w) blends. M = Zn2+, Mg2+, Li+, or Na+. The blending temperatures were denoted in the figure legends. Figure 1. Effect of blending temperature on impact strength of PLA/ EBA-GMA/EMAA-M (80/10/10, w/w) ternary blends. M = Zn2+, Mg2+, Li+, or Na+.
EMAA-Zn (or EMAA-Mg) than EBA-GMA in the ternary blends. Again, the toughening effects of EMAA-Na and EMAALi are minimum. The difference in toughening effect between monovalent and divalent metal ion ionomers is attributed to their different activities in promoting interfacial compatibilization and cross-linking reactions during compounding of the ternary blends, as will be discussed in the subsequent sections. PLA is susceptible to thermal degradation and hydrolysis. The change of PLA molecular weight after melt-processing for different ternary blends was examined using GPC (Table 1). The degree of PLA thermal degradation varied with the ionomers used in the following order: EMAA-Zn < EMAA-Mg < EMAA-Li < EMAA-Na. When blended at 240 °C, PLA displayed decreases of 9.5 and 14.8% in the number-average molecular weight of PLA (Mn, PLA) for the ternary blends with EMAA-Zn and EMAA-Mg, respectively, compared to that of the neat PLA control (Mn, PLA = 101 700 g/mol) prepared under the same conditions. In contrast, when prepared at 210 °C, PLA displayed decreases of 50.8 and 61.2% in Mn for the ternary blends with EMAA-Li and EMAA-Na with respect to neat PLA control (Mn, PLA = 107 200 g/mol) prepared at 210 °C, respectively. Clearly, monovalent ion ionomers caused much more severe thermal degradation of PLA than divalent ion ones. Fan et al. compared the effects of the two alkali earth metal oxides (CaO vs MgO) on thermal degradation of PLLA and noted that CaO resulted in a much lower degradation temperature and more racemization of lactide during the pyrolysis than MgO.22 They attributed it to the higher basicity of Ca2+ over Mg2+. Therefore, one possible reason for the difference between the two types of ionomers in inducing PLA degradation was that Li+ and Na+ ions had higher basicity than Mg2+ and Zn2+. By examining the effects of four residual metals on thermal stability of PLLA, Cam and Marucci found that the degradation effect increased in the following order: Fe > Al > Zn > Sn.23 They argued that the higher the selectivity of catalyst used for lactide polymerization, the less efficient the depolymerization at high temperature. It has been recognized that the oxides, halides, and alkoxides of Mg and Zn which possess free p, d, or f orbitals, are able to effectively catalyze the ring-opening polymerization of lactide through a coordination− insertion mechanism.29,30 Thus, high catalytic activity of Zn and Mg compounds on lactide polymerization might be another
notched Izod impact strengths (IS) of PLA ternary blends containing different types of ionomers. It was found that the IS of the ternary blends varied drastically with blending temperature and metal ion type of ionomer. EMAA-Zn exhibited the highest toughening effect among these ionomers. As the blending temperature was increased from 185 to 210 °C, the resulting ternary blend with EMAA-Zn showed a drastic increase in IS. The ternary blend prepared at 210 °C had an IS of 737 J/m. A further increase in the blending temperature to 240 °C only led to a slight increase in IS (777 J/m). In contrast, EMAA-Mg displayed little toughening effect (IS of 25 J/m and 54 J/m, respectively) at the blending temperatures of 185 and 210 °C. Only when the blending temperature was raised to 240 °C, did EMAA-Mg exhibit a remarkable toughening effect, and the resulting blend had an IS of 335 J/m. On the other hand, EMAA-Li and EMAA-Na showed almost no toughening effect irrespective of blending temperature. For example, the blends with EMAA-Li or EMAA-Na prepared at 210 °C showed an IS of 47 J/m and 27 J/m, respectively, compared to 23 J/m for the neat PLA prepared under the same conditions. This level of toughness was also much lower than that of the corresponding ternary blend with the non-neutralized EMAA-H precursor (97.4 J/m).15 When the blending temperature was further increased to 240 °C, EMAA-Na and EMAA-Li resulted in severe thermal degradation of PLA as the extrudate appeared very fluid. For these reasons, the test specimens were not prepared from those two blends. This result suggests that the EMAA-derived ionomers with monovalent sodium or lithium ions had no effect in promoting impact toughness. Besides, tensile strength and modulus of the ternary blends with different ionomers only differed slightly and lay between 34 and 38 MPa and between 2.1 and 2.4 GPa, respectively (curves not shown). Figure 2 shows the toughening effect of elastomer/ ionomer weight ratio at a fixed total content of elastomer and ionomer (20 wt %). It was noted that the high toughening effect was achieved only when the weight fraction of EBA-GMA was larger than that of EMAA-Zn (or EMAA-Mg). The toughening effect deteriorated quickly when there were more 4789
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Table 1. Physical and Morphological Properties of Different PLA/EBA-GMA/EMAA-M Ternary Blends system PLA/EBA-GMA/EMAA-Zn (80/10/10, w/w) PLA/EBA-GMA/EMAA-Mg (80/10/10, w/w) PLA/EBA-GMA/EMAA-Li (80/10/10, w/w) PLA/EBA-GMA/EMAA-Na (80/10/10, w/w)
extrusion temp (°C) 240 240 210 210
Mn,PLA (g/mol) e
92 000 86 600 52 700 41 600
PDIa 1.31 1.30 1.22 1.28
e
APLA/AEBA‑GMAb 0.48 0.23 0.09 0.08
f
dwc (μm) 0.88 1.20 1.72 1.47
f
dw/dnc 1.77 2.48 3.11 2.71
f
Xc,PLAd (%) 1.0e 3.0 4.7 3.4
Polydispersity index, PDI = Mw/Mn. bAPLA/AEBA‑GMA: the ratio of absorption peak area at ∼1759 cm−1 to that at ∼1734 cm−1. cdw = Σnidi2/Σnidi, dn = Σnidi/Σni. dThe crystallinity of PLA (Xc,PLA) in the injection molded specimens was determined from the first DSC heating scan at 10 °C/min. e The data were cited from ref 17. fThe data were cited from ref 16. a
Figure 3. SEM images of cryo-fractured PLA/EBA-GMA/EMAA-M (80/10/10, w/w) ternary blends: (a) M = Zn2+ and blending temp = 240 °C; (b) M = Mg2+ and blending temp = 240 °C; (c) M = Li+ and blending temp = 210 °C; (d) M = Na+ and blending temp = 210 °C.
possibility to account for the less pronounced effects of Zn and Mg ionomers on PLA degradation in the study. 3.2. Reactive Interfacial Compatibilization and CrossLinking Reaction. It is necessary to clarify the effects of metal ion type of the ionomers on interfacial compatibilization and cross-linking reactions, since both reactions were found to affect the toughness of the ternary PLA blends.16 Figure 3 shows the SEM images of cryo-fractured blends containing different ionomers. For the ternary blends containing EMAANa and EMAA-Li, the smooth surfaces of the dispersed particles or the pits left by the particles were clear evidence of poor interfacial adhesion. In contrast, improved interfacial adhesion and wetting were evident in the ternary blends containing EMAA-Zn and EMAA-Mg. Furthermore, the increased compatibilization was also reflected in the fine dispersion of the particles in the two blends. This morphological result, along with the above impact properties of the blends, indicates that the high impact performance of the ternary blends was closely associated with the strong interfacial compatibilization and strong adhesion. To further evaluate interfacial compatibilization, FT-IR analysis of the residues from dioxane-extracted ternary blends was employed (Figure 4). The absorption peaks at ∼1759 and ∼1734 cm−1 resulted
Figure 4. FT-IR absorption spectra of residues from dioxane-extracted PLA/EBA-GMA/EMAA-M (80/10/10, w/w) ternary blends. M = Zn2+, Mg2+, Li+, or Na+. Note: the curve with EMAA-Zn and 240 °C was cited from ref 16.
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from the stretching vibration of carbonyl groups in the PLA and EBA-GMA components, respectively. The ratio of the absorption peak area at ∼1759 cm−1 to that at ∼1734 cm−1, APLA/AEBA‑GMA was used to approximately estimate the extent of reactive compatibilization between PLA and EBA-GMA.16 For the four ternary blends containing different ionomers, the APLA/AEBA‑GMA ratio followed the same order as these ionomer influenced the IS, that is, EMAA-Zn > EMAA-Mg > EMAA-Li ≈ EMAA-Na (Table 1). The kinetics of the EBA-GMA cross-linking reaction was studied using a torque rheometer. Figure 5 shows torque
Figure 5. Torque versus mixing time for compounding of various binary EBA-GMA/EMAA-M (50/50, w/w) blends at 50 rpm. Compounding temperature for EBA-GMA/EMAA-Zn (or -Mg) was set at 240 °C and for EBA-GMA/EMAA-Na (or Li) at 210 °C.
evolution as a function of mixing time for various EBA-GMA/ EMAA-M binary blends (50/50, w/w). The torque rose again after the initial peak associated with polymer melting, confirming the occurrence of cross-linking for all binary blends. The peak height can be approximately related to the extent of reaction and the slope of torque increase to reaction rate, while the peak time is associated with reactivity. It was clearly shown that cross-linking reaction of EBA-GMA with zinc or magnesium ionomer proceeded at much faster rates and higher reactivity than with sodium or lithium ionomers. Especially, the peak torque value in the mixing with EMAA-Zn was much higher than that in the mixing with the remaining three ionomers, suggesting that the cross-linking reaction in the former proceeded to a much higher degree. As the mixing continued, the cross-linked EBA-GMA was crumbed gradually and therefore the torque was seen to decrease accordingly from the peak value. It was noted that the maximum torque for mixing with EMAA-Zn was achieved at ∼2.5 min, indicating the cross-linking reaction could be completed in a few minutes. It is well-known that dynamic mechanical properties of polymer materials are very sensitive to the molecular or segmental mobility. Figure 6 shows the dependences of storage modulus (E′) and damping factor (tan δ) on blending temperature. In Figure 6a, E′ in the glass region differed slightly between the ternary blends with different ionomers. Also, no perceptible change in the Tg (∼68 °C) of the PLA matrix was noted between the different ternary blends. However, the Tg of the rubbery EBA-GMA phase differed markedly from blend to blend (Figure 6b). By using EMAA-Mg and EMAA-Zn for the blends, the Tg of the EBA-GMA phase
Figure 6. Storage modulus (a) and damping factor (b) as functions of temperature for various PLA/EBA-GMA/EMAA-M (80/10/10, w/w) ternary blends. M = Zn2+, Mg2+, Li+, or Na+.
was increased ∼3 and ∼6 °C, respectively, with respect to using EMAA-Na (or −Li). The change of Tg of the EBA-GMA phase was due to the difference in its cross-linking level between different ternary blends.31 The result of Tg varying with the metal ion type was in agreement with that of torque rheology. 3.3. Dispersed Phase Morphology and Crystallinity of PLA Component. Figure 7 shows TEM images of the ternary blends containing different ionomers. The ternary blends prepared with monovalent and divalent metal ion ionomers exhibited different “particle-in-particle” morphologies. Both the ternary blends with EMAA-Zn and EMAA-Mg displayed a welldefined “salami-like” phase structure (see insert). According to our previous work,16 the dark subdomains in the dispersed particles were the ionomer-rich regions containing metal ions that had higher electron density than PLA and EBA-GMA. It appeared that EMAA-Zn was more uniformly dispersed inside the irregular elastomer particles than EMAA-Mg that still existed in the form of some large and dark subdomains (indicated by the arrow), suggesting that the former presumably had better reactive compatibility with EBA-GMA. In contrast, the “core−shell” dispersed particle structure was predominant in the ternary blends with EMAA-Na and EMAALi. From the above analysis of torque rheology, zinc or magnesium inonomer exhibited higher reactivity with EBAGMA than sodium or lithium one. Therefore, such single-core 4791
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Figure 7. TEM images of PLA/EBA-GMA/EMAA-M (80/10/10, w/w) ternary blends. (a) M = Zn2+ and blending temp = 240 °C; (b) M = Mg2+ and blending temp = 240 °C; (c) M = Li+ and blending temp = 210 °C; (d) M = Na+ and blending temp = 210 °C.
dominant for the ternary blends with the divalent ionneutralized ionomer (EMAA-Zn or -Mg), while a “core− shell” structure is formed for the blends with the monovalent ion-neutralized ionomer (EMAA-Li or -Na). The findings from this study provide useful guidance to the reactive toughening of PLA or other brittle biodegradable polyesters.
substructure for the ternary PLA blends with the monovalent metal ion ionomer might be attributed to the low reactivity of Na and Li inomers with EBA-GMA, thereby resulting in a relatively coarse dispersion of the subdomains. The calculated particle size and distribution are summarized in Table 1. It appeared that the ternary blends with sodium and lithium ionomers had relatively larger weight average particle diameters (dw) and size distributions (dw/dn) than those with the magnesium and zinc ionomers. Since it was also reported that crystallization of the PLA matrix would influence the impact properties of PLA blends,6,7,10,11,13,14 the crystallinity of PLA in the injection molded specimens was also measured from the first DSC heating scan, and the results are given in Table 1. All ternary blends exhibited the crystallinity of less than 5%. Therefore, such small crystallinity of the PLA matrix hardly accounts for the remarkable difference in impact toughness between the ternary blends with different ionomers.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (509) 335-8723. Fax: (509) 335-5077. E-mail: jwzhang@ wsu.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support from the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, Grant No. 2007-3550417818. We also appreciate the kind assistance in the electron microscopic observation from the staff in the Franceschi Microscopy and Imaging Center (FMIC), Washington State University.
4. CONCLUSIONS Metal ion type of EMAA-derived ionomers greatly influenced the impact toughness of PLA ternary blends. Of all the ionomers used, the zinc ionomer had the highest toughening effect for the ternary blends, followed by the magnesium ionomer, while the sodium or lithium ionomers had little influence on the toughness of the ternary blends. Blending temperature and elastomer/ionomer ratio also exhibited great influences on the toughness of the ternary blends containing zinc or magnesium ionomer. With zinc and magnesium ionomers, the toughening effect increased with blending temperature. Zinc ionomer exhibited higher catalyzing effects than the other ionomers in promoting the interfacial compatiblization of PLA/EBA-GMA and cross-linking reaction of the elastomer. Unlike the zinc or magnesium ionomer, lithium or sodium ionomers induced more severe thermal degradation of the PLA matrix during compounding. The metal ion type also had a distinct effect on the “particle-in-particle” morphology of ternary blends. A “salami-like” structure is
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REFERENCES
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dx.doi.org/10.1021/ie303317k | Ind. Eng. Chem. Res. 2013, 52, 4787−4793
